edje: make functions return Eina_Bool and fix documentation.

edje: make functions return Eina_Bool and fix documentation.

Make the following functions return Eina_Bool so the caller can detect errors:
edje_edit_state_font_set
edje_edit_part_effect_set

Also deleted duplicate of the "edje_edit_state_font_set" function in Edje_Edit.h
Moved some defines (EDJE_TEXT_EFFECT_MASK_BASIC, EDJE_TEXT_EFFECT_MASK_SHADOW_DIRECTION etc),
so doxygen generation was fixed.
Also added link (see also) in "edje_edit_part_effect_set" to the Edje_Text_Effect enum.

Reviewers: cedric, seoz

CC: reutskiy.v.v
Differential Revision: https://phab.enlightenment.org/D302

Signed-off-by: Cedric Bail <cedric.bail@samsung.com>